Since image data, especially moving image data, is large in volume, a high efficiency encoding is performed when the data is transmitted from a transmission apparatus to a receiving apparatus, or when the data is stored in a storage device. The “high efficiency encoding process” refers to a process of transforming a data sequence into another data sequence, and compressing the volume of data.
One of a method for a high efficiency image encoding process is a prediction encoding. The prediction encoding calculates a difference between each encoding target block obtained by dividing an encoding target image into a plurality of blocks and a corresponding prediction image generated based on a locally decoding process, and generates a prediction error image. The frequency coefficients obtained by performing an orthogonal transformation on the prediction error image is quantized, and resultant quantized coefficients are encoded and transmitted to the decoding side.
In addition, since large portion of coefficients obtained by converting an image into a frequency domain are distributed in low frequency area, an entropy coding may be performed.
However, with recent high image quality, a higher efficiency image encoding technique is demanded.
As related art, following documents are known.    (1) Patent Document 1: U.S. Pat. No. 2,632,058    (2) Non-patent Document 1: Academic Papers of the Institute of Electronics, Information and Communication Engineers B Vol. J65-B No. 1 pp. 23-30, “Method of Designing a Pixel Adaptive DPCM Encoding based on the region segmentation of a screen”    (3) Non-patent Document 2: Ian H. Witten et. al., “Arithmetic coding for data compression, Communication of the ACM, June 1987, Vo. 30, No. 6, pp. 520-530”